Mistake #1 - Trusting Presets.
The plugin designers have absolutely no idea what your track sounds like, so their frequency boost and cut recommendations are completely irrelevant to you and your mix.

Mistake #2 - Focusing On What The EQ Curve Looks Like.
Your eyes can trick you into hearing things that aren't there. Try to close your eyes when bypassing your EQ plugins to make sure that it SOUNDS better and doesn't simply LOOK like it's EQ'd properly.

Mistake #3 - Assuming Each Track Needs EQ.
If a track sounds good in the mix and sits well with everything else, it doesn't need any EQ. The less processing you can do to your mix, the more natural and musical it will sound - so avoid EQing just because you feel you should. Only touch a track if it needs help.[/list]